It is well known that back pain is one of the most frequently occurring and expensive disabling ailments, especially for patients in the 30–60 year age bracket. Although back pain syndrome is a very common occurrence, its diagnosis to this day is very difficult.
The vertebral column (spine) is a biomechanical structure composed primarily of ligaments, muscles, vertebrae and intervertebrae discs. The biomechanical functions of the spine include (1) support of the body (trunk and appendages), which involves the transfer of the weight and the bending moments of the head, trunk and arms to the pelvis and legs, (2) complex physiologic motion between these body parts, and (3) protection of the spinal cord and nerve roots.
The major regions of the spine are the cervical, thoracic, lumbar and sacral. The vertebrae increase in size and mass from the cervical to the lumbar regions. The increase in size of the vertebrae is directly related to an increased capacity for supporting larger loads. The lumbar region is therefore the major load bearer of the spine. However, this increase in load bearing capacity is paralleled by a decrease in flexibility. Because the lumbar regions bears heavier loads than other regions of the spine, the lumbar trunk (low back structure) is more, but not exclusively, susceptible to strain and hence back pain.
The spine is comprised of different levels known as motion segment units. The lumbar spine, for example, is comprised of five motion segment units. The motion segment unit is the smallest component of the spine that exhibits kinematic behavior similar to that of the whole spine. The motion segment unit is capable of flexion, extension, lateral bending, torsion and translation. The components of each motion segment unit include two adjacent vertebrae and their apophyseal joints, the intervertebral disc and the connecting ligamentous tissue.
Many causes of back pain and related neurological pain, are attributed to the instability of the motion segment unit. Segmental instability is defined as “the loss of ability of the spine under physiologic loads to maintain relationships between vertebrae in such a way that there is neither damage nor subsequent irritation to the spinal cord or nerve roots, and, in addition, there is no development of incapacitating deformity or pain due to structural changes”. Instability is therefore an abnormal response to applied loads characterized by motion in the motion segment unit beyond normal constraints. Excess motion can be abnormal in quality (i.e., abnormal coupling patterns) or in quantity (abnormal increased motion) or both. Excess motion may well result in damage to the nerve roots, the spinal cord, and other spinal structures.
The underlying causes of the structural changes in the motion segment unit leading to instability are trauma, degeneration, aging, disease (tumor, infection, etc.), surgery, or a combination thereof. It is known that a mechanically unstable motion segment unit can originate due to loss of biomechanical function of the spine joint ligaments and degeneration of the intervertebral disc and nucleus pulposus. A degenerate nucleus polposus causes disc space narrowing, loss of viscoelastic properties and the subsequent transfer of compressive loads to the annulus fibrosus. The altered anatomic dimensions and subsequent abnormal response to loading can cause loss of pretension in the ligamenum flavum, and longitudinal ligaments, degeneration of the facet capsules (and possible subluxation) with a consequence of secondary degenerative osteoarthritis of the joints.
Spinal disorders requiring neural decompressive surgery can leave motion segment units unstable due to the removal of supporting structures of the joint. A severely unstable motion segment unit is most likely to be fused to insure postsurgical stability. The need to fuse the vertebrae of a motion segment unit is dependent on the pre-operative symptoms and clinical (radiographic) findings and on the outcome of the surgical procedure.
One effort at mechanically determining spinal instability is disclosed in “A Technique for Mechanical Assessment of the Intervertebral Joint”, Mark Lubin et al., Biomech. Sym. ADM vol. 43 (1981). A Cloward lamina spreader is fitted with a strain gauge and a loading and unloading of force is provided manually. The device disclosed in the aforementioned publication is disadvantageous because there is no recognition of the need to control the rate of displacement nor a means for doing so which enables precise measurements of relative stiffness of the motion segment unit. The motion segment unit is a viscoelastic structure and therefore its resistance to deformation is dependent on the loading rate. Objective criteria for determining the degree of instability of the motion segment unit is therefore important in assessing whether spinal fusion surgery is necessary to relieve back pain in the patient.
Another effort at measuring the relative instability of the motion segment unit of the spine is disclosed in Mark D. Brown and David C. Holmes (U.S. Pat. No. 4,899,761). The apparatus disclosed in this reference provides a vertebrae distractor including a device for applying a constant rate of increasing force against adjacent vertebrae of a motion segment unit to thereby distract or separate the vertebrae. Means for detecting and recording the changes in the resistance to distraction are also provided. The device disclosed in the '761 Patent, while providing useful objective criteria regarding the relative stiffness of a motion segment unit of the spine, nonetheless, requires the removal of spinal tissue in order to place the distractor legs in a suitable position for operating the device as shown in FIG. 2 of the reference. In particular, it is often necessary to remove the interspinous ligaments from adjacent vertebrae in order to provide placement of the distractor device in an operable position to measure spinal stiffness. The removal of spinal tissue with this procedure may contribute to the instability of the motion segment unit. Thus, the surgeon must first further destabilize the motion segment unit before a measurement can be taken and this may have a bearing on the type of implantable spinal assist device that may be used to correct the instability and the degree to which the patient may recover from the spinal surgery.
It would therefore be a distinct advantage in the art for measuring and treating instability of a motion segment unit of the spine if a device used to determine the relative stiffness of a motion segment unit did not result in significant damage and/or removal of spinal tissue in order to make the appropriate measurements of spinal stiffness.
It would be a further advantage in the art to provide a device for measuring spinal instability which can be readily attached to preselected positions of the motion segment unit during operation without significant tissue damage.
It would be a still further advantage in the art to provide a device for measuring spinal instability which can be employed in a comprehensive system in which spinal stiffness or other characteristics of the motion segment unit can be matched with a suitable spinal assist device such as a spinal implant device for reducing or eliminating instability of the motion segment unit.